1. Field of the Invention
The present invention relates in general to multiplexed fiber optic sensor arrays with large spans (coverage) that can be used for a number of surveillance type applications, and more particularly to ladder-type sensor arrays having a forward coupled topology.
2. Description of Related Art
A wide range of network architectures for multiplexing fiber optic sensors using time and frequency division techniques have been developed. Most fiber optic sensor multiplexing architectures, or topologies, involve the use of fiber optic couplers or splitters to distribute the light to, and recombine the light returned from, the sensor elements in the array. In a typical ladder-type network, the losses in the system can become large, and the coupling or splitting ratios in the couplers or splitters require adjustment to compensate for such losses.
One form of conventional ladder network topology is the return-coupled array topology (RCAT). As shown in FIG. 1, in an RCAT ladder network, a plurality of forward-coupling fiber optic distribution couplers (power splitters) 1A are connected by first optical fiber sections 4A in series, a plurality of return-coupling fiber optic recombination couplers 1B are connected by second optical fiber sections 4B in series, a plurality of fiber optic sensors S.sub.1 -S.sub.n are respectively connected between corresponding ones of the first and second couplers 1A and 1B, and a further terminal fiber optic sensor S.sub.t is series connected with terminating ones of the first and second optical fiber sections 4A and 4B, respectively, as shown in FIG. 1. As shown in more detail in FIG. 2, couplers 1 are joined to the optical fiber sections 4 by splices 3; and delay coils T having a length L may be connected in series with the optical fiber sections 4.
In general, when dealing with array topologies based on an RCAT ladder network, the coupling or splitting ratios .kappa..sub.1 -.kappa..sub.N of the couplers 1 can be selected to equalize the power returned to a detector (not shown) from each sensor. This equalization optimizes the overall array performance in terms of signal to noise ratio. The values of the required splitting ratios .kappa..sub.x depend on the number of optical elements in an array and the light losses in the optical elements. The total number of sensors that can be supported is also determined by the light losses. The splitting ratios .kappa..sub.x for an ideal RCAT ladder network without light losses are determined by EQU .kappa..sub.x =1/(N+1-x) (1)
where N is the number of input and output fiber optic couplers and x=1 to N-1.
Light losses in couplers 1, splices 3, and delay coils T reduce the optical power received from different sensors 2 unequally. The farther a sensor S is from the common input/output terminals of the array, the greater are the light losses through the optical elements because of the greater number of optical elements through which the light passes.
Because the light losses accumulate, based on the number of optical elements through which the light passes, even if the losses .sup..about. -0.1 dB, in couplers 1, splices 3, and delay coils T are the light loss for the last sensor of an eight element RCAT ladder network can total more than -5 dB. Power balancing is then achieved by lowering the coupling ratios of the initial couplers 1 in both the distribution and recombinational coupler networks. The required fractional reduction in coupling ratio depends on the losses and the position of each coupler 1 in the RCAT ladder network.
The effect of light losses in the optical elements of the RCAT ladder network can be analyzed in terms of a unit segment or section of the ladder network array that is repeated sequentially along the RCAT ladder network, for example, a unit segment associated with a sensor S.sub.1 as shown in FIG. 2. The unit segment includes the sensor S.sub.1 (e.g., a hydrophone), the associated distribution and recombination couplers 1A and 1B, delay coil T of length L, connecting sections of optical fibers 4, and the associated splices 3 as indicated. The losses in the couplers 1, splices 3, delay coils T, and the total length of the connecting optical fiber sections 4 are denoted .gamma..sub.c, .gamma..sub.s, .gamma..sub.d, and .gamma..sub.l, respectively. The losses .gamma..sub.d of each delay coil T can be expressed as .gamma..sub.f L, where .gamma..sub.f is the light loss of the optical fibers per unit length and L is the length of the delay coil T. Typically, the length of delay coil T may be .sup..about. 25 m, much greater than the length of the connecting optical fiber sections, and therefore the light loss .gamma..sub.l may be so small as to be ignored. In that case, the total loss of each coupler and its associated splices and delay coil is given by the equation EQU .gamma.=.gamma..sub.c +2.gamma..sub.s +.gamma..sub.f L (2).
However, in certain applications, an RCAT ladder array may require sensor spacing of 100 m to kms, in which case the loss in the connecting fibers can dominate the other light losses in determining the required coupling ratios. Therefore, in those application, the light loss .gamma..sub.l cannot be ignored.
FIG. 3 shows the required coupling ratios, determined by numerical analysis, for a sixteen sensor RCAT ladder array with optical elements having different light losses. The bold curve, with squares designating the array sensors, shows the coupler splitting ratios for the ideal "lossless" case. The curves with triangle and circle sensor designations show the required coupler splitting ratios for coupler, splice and link losses of -0.3 dB and -1.0 dB, respectively, associated with each sensor.
The effective throughput per sensor channel within the RCAT ladder array assembly, which can be calculated using the data of FIG. 3, are shown in FIG. 4. Of the total effective sensor loss shown in FIG. 4, -0.3 dB is due to the output coupler of the sensor. On a time averaged basis, only 50% of the optical power from a sensor is directed to the output port coupled to the series of recombination couplers. The bold curve, with squares designating the array sensors, shows the optical throughput per sensor channel for the ideal "lossless" case without coupler, splice and link losses. The curve in FIG. 4 with triangle sensor designations shows that for a sensor loss of -1 dB, and coupler, splice and link losses of -0.3 dB, the effective optical throughput per sensor channel is approximately -34 dB. The curve with circle sensor designations shows that adding a fiber link loss of -0.7 dB, which corresponds to a 2 km spacing at an optical wavelength of 1.3 .mu.m, decreases the optical throughput per sensor channel to -50 dB.
With the coupler splitting ratios selected according to FIG. 3, efficient power distribution/recombination of the sensor signals within the array is achieved. In response to the input optical pulse, the array produces a set of N equal magnitude output pulses separated by the sensor-sensor delay period.
As indicated in FIG. 3, the RCAT ladder network requires that the coupler splitting ratio of each coupler be chosen to compensate for light losses of the various optical elements.
Additionally, the couplers near the input/output end of the array must have very low coupler splitting ratios, which is difficult to achieve with a small tolerance. Errors in those splitting ratios cause errors in the optical throughputs of all the sensor channels farther away from the input/output of the array.